The polymeric aromatic carbamates (polyurethanes), such as the diphenylmethane dicarbamates and related higher homologs, polymethylene polyphenyl carbamates, and especially the diethyl esters, have become increasingly important products, particularly for use in the preparation of the commercially valuable diphenylmethane diisocyanates and mixtures of diisocyanates and polyisocyanates by the thermal decomposition of such polymeric aromatic carbamates in a suitable solvent as shown, for example, in Rosenthal et al, U.S. Pat. Nos. 3,962,302 and 3,919,279.
Processes have been proposed for the preparation of polymeric aromatic carbamates (polyurethanes), as for example in Klauke et al, U.S. Pat. No. 2,946,768 and British Pat. No. 461,352, which disclose the condensation of aryl carbamic acid esters with carbonyl compounds such as aldehydes and ketones in a dilute aqueous mineral acid medium, and in co-pending U.S. application, Ser. No. 953,135, filed Oct. 20, 1978, now Pat. No. 4,162,362, which describes the condensation of N-aryl carbamic acid esters with formaldehyde, para-formaldehyde or trioxane and an organic sulfonic acid catalyst which has an acid concentration of at least 75 percent. In the Klauke et al U.S. Pat. No. 2,946,768 and British Pat. No. 461,352 processes, large amounts of undesirable N-(alkoxycarbonyl)phenylaminomethylphenyl compounds, referred to as N-benzyl compounds, are formed in addition to desired diphenylmethane dicarbamates and polymethylene polyphenyl carbamates. The rearrangement or conversion of the "N-benzyl" compounds to the desired carbamates is disclosed and fully described in co-pending U.S. application, Ser. No. 905,705, filed May 15, 1978, now U.S. Pat. No. 4,146,727.
The diphenylmethane dicarbamates in the total carbamate product resulting from any of the above noted processes generally range from about 20 to 85 percent and are a mixture of the 4,4', 2,4' and 2,2' isomers; the remainder of the carbamate product being trimers, tetramers, etc. of such compounds. Of the diphenylmethane dicarbamates in the carbamate product, the 4,4', to the 2,4' and to the 2,2' isomer ratio is approximately 4:1 and 20:1 respectively or approximately 78 percent 4,4', 18 percent 2,4' and 4 percent 2,2'. Such a product mixture is generally useful for thermal decomposition in a solvent to produce the commercially valuable diphenylmethane diisocyanates having the same isomer ratio, which isocyanates may be used to prepare urethane foam products when reacted with glycols. However, for certain applications, for example, in elastomer and coating applications or to provide greater reactivity in injection molding, it is particularly desirable to have a dicarbamate product, which when produced by the condensation processes as hereinabove noted, is enriched in or has a larger amount of the 4,4'-diphenylmethane dicarbamate isomer, which upon such decomposition in solvent provides an isocyanate enriched in the 4,4'-diphenylmethane diisocyanate.
The present invention comprises the preparation of diphenylmethane dicarbamates and polymethylene polyphenyl carbamate homologs and derivatives of these compounds by the acid catalyzed condensation of N-aryl carbamates with formaldehyde, para-formaldehyde or trioxane in a single phase inert solvent system which includes a method for controlling the disubstituted isomer to produce a product enriched in 4,4'-diphenylmethane dicarbamates, especially, the 4,4'-diphenylmethane dicarbamate, diethyl ester, as compared to the 2,4' and 2,2' isomer.